The recognition of MHC-peptide ligands by the T cell receptor (TCR) is central to the induction of the adaptive immune response. This thesis describes the development of a bioassay for TCR recognition which was then used to undertake a molecular analysis of the TCR/MHC-peptide interaction. 1. A TCR-CD3ϛ chimeric receptor was stably expressed in the cell line RBL-2H3 to give the transfectant RBL-008. RBL-008 was shown to exhibit MHC-restricted peptide-specific responses to both cellular and multimerised recombinant HLA-A2-pol peptide targets (Chapter 3). 2. By competitively inhibiting the response of RBL-008 to HLAA2 pol complexes with monovalent soluble recombinant MHCpeptide complexes it was confirmed that the TCR makes significant contact with both the MHC and peptide parts of its ligand. Furthermore it was found that only a few peptides in a random mixture can prevent contact between the TCR and HLA-A2. This has implications for positive selection since it supports evidence suggesting that some TCRs can be selected on a wide range of unrelated peptides (Chapter 4). 2. The bioassay was used to examine the flexibility of TCRpeptide interactions using a panel of variant peptides designed on the basis of the previously published HLA-A2-pol peptide structure (Chapter 5). Several variant peptides were recognised by the TCR and interestingly one of these altered peptide ligands was actually recognised better than the index peptide, raising the prospect of designing 'improved epitopes'. 3. By mutating the β chain of TCR-CD3ϛ chimeric receptor it was shown that allelic variation in the TCR genes can have a significant effect on antigen recognition and may therefore be disease susceptibility candidates genes (Chapter 6). 4. The structural relationship between the V and C domains of the TCR was examined and found to be of considerable functional significance since disruption of this relationship resulted in loss of expression of the TCR-CD3ϛ receptor.